The physical chemical properties of fatty acid mixtures spread as monomolecular films is being exained to observe the tendency for dissimilar molecules to mix and interact when part of oriented monolayers. This simulates the oriented structure of biological membranes. Techniques used and theories applied will allow determination of the degree of miscibility or immiscibility and, in the case of miscible molecules, the extent of deviation from ideal non-specific mixing. From observed deviations it will be possible to estimate activity coefficients for the miscible systems, and a quantitative estimate of tendencies to "mix" or "demix." Since the only variable chemically is the fatty acid chain, quantitative estimates of fatty acid chain compatibility will be determined. Structural changes include: chain length, unsaturation, degree of unsaturation, position of double or triple bond, and presence of hydroxy groups on the chain at different positions. The relationship of these observations to fluidity in biological membranes will be evaluated.